The function of a polarizing beamsplitter (PBS) is to reflect light in one polarization state and to transmit light in the orthogonal polarization state. Consequently, PBSs find widespread use in optical systems that rely on the polarization of the light. An example of one such system is an image projection system that uses a reflective liquid crystal display (LCD) panel for modulating an illumination light beam: a polarized illumination light beam is directed to the LCD panel, for example by reflection in the PBS. The light beam is spatially modulated by the LCD panel so that the reflected beam contains some unmodulated light in the polarization state of the illumination beam and some modulated light in the orthogonal polarization state. The unmodulated, non-image light is reflected by the PBS and the modulated, image light, which contains the desired image, is transmitted through the PBS. Thus, the PBS separates the image light from the non-image light and the image light can then be projected to a screen for viewing by a user.
PBSs are often formed as a polarizing layer sandwiched between the hypotenuses of two right-angled, glass prisms. Exemplary polarizers used in a projection system can include, for example, MacNeille PBSs, which rely on a stack of quarter wave films of isotropic material oriented at Brewster's angle for one of the polarization states as the polarizing layer, or Cartesian multilayer optical film (MOF) PBSs, which use a stack of alternating isotropic and birefringent polymer materials as the polarizing layer.
If there is any birefringent retardation in the glass prism portion of the PBS, the effectiveness of the PBS can be reduced because, for example, the nominally s-polarized illumination light reflected from the polarizing surface is rotated, due to the birefringence of the glass prism, to being partially p-polarized. When used in a liquid crystal on silicon (LCoS) type imager, undesirable rotation of the polarization can occur both on the light path to the imager and on the light path after being reflected by the imager. Rotation of the light both entering and exiting the imager can cause undesirable light to leak through the polarizing layer thereby reducing contrast of the projection system. The reduction in contrast is due to an increase in the level of brightness in the dark state. Birefringent retardation in the glass prism may result from a number of different causes, for example, mechanical stresses induced in the PBS components while assembling the PBS, stresses induced by attachment of the PBS, or by thermal expansion in the PBS when subjected to an intense illumination light beam.
In an effort to overcome this problem, a significant amount of work was done by glass manufacturers to make glass that develops very little birefringence in response to mechanical stress. PBH56 made by Ohara Inc., having headquarters in Japan, and SF57 made by Schott AG, having headquarters in Germany, are example glasses of this typeand. Such glasses contain lead (chemical symbol Pb) in significant quantities and therefore are not environmentally desirable materials. These materials can also be expensive and difficult to process. Additionally, the low SOC glasses have a high refractive index, in excess of 1.8, which may lead to optical inefficiencies or aberrations when matching to the lower refractive index polarizing layers.
Due to increasingly strict environmental laws related to the use and disposal of lead-based materials, the use of lead in the glass portion of the PBSs is becoming less common. Without the use of lead in the PBS, the PBS can be highly sensitive to mechanical stresses that result in birefringence and other performance degrading issues.